A recognized way of reducing the feature size of circuit elements on microchips is to lithographically image them with radiation of a shorter wavelength. This is clear from the well-known relationship ##EQU1## where R is the resolution dimension representing feature size, K is a process-related constant of order unity, .lambda. is the wavelength of the radiation, and NA is the numerical aperture of the imaging system projecting the radiation onto a wafer. Shortening the wavelength .lambda. thus directly reduces the resolution dimension.
Enlarging the numerical aperture, as another way of reducing the resolution dimension, also reduces the depth of focus (Df), by the relationship ##EQU2## For several reasons, including the practical flatness of wafers, depth of focus is preferably larger than about 1.0 micron, which in turn limits the resolution improvement achievable by enlarging the numerical aperture. This leaves shortening the wavelength of the radiation as the most desirable way of improving resolution, providing ways can be found for distortion-free imaging with shorter wavelengths.
Moving down the electromagnetic spectrum to wavelengths shorter than UV radiation leads to the so-called "soft" X-ray radiation in the range of 2 to 20 nanometers wavelength. Radiation in the soft X-ray range cannot be focused refractively by passing through glass lenses, but can be focused by reflective mirrors having multilayer coated surfaces. This possibility has led to some work on soft X-ray imaging systems using mirrors in a projection imaging lens system. Examples of such work include:
The basic problem is well explained by H. Kinoshita et al. in their paper "Soft x-ray reduction lithography using multilayer mirrors" (J. Vac. Sci. Technol. B 7 (6), Nov/Dec 1989, pages 1648-1651). PA1 One of the inventors of this application (J. H. Bruning) has contributed to a paper entitled "Reduction imaging at 14 nm using multilayer-coated optics: Printing of features smaller than 0.1 .mu.m" (J. Vac. Sci. Technol. B 8 (6), Nov/Dec 1990, pages 1509-1513). PA1 A workshop on this subject, High-Precision Soft X-ray Optics Workshop, held Oct. 5 and 6, 1989, was sponsored by the Air Force Office of Scientific Research and the National Institute of Standards and Technology. A notebook entitled "High-Precision Soft X-Ray Optics" from this workshop includes a section on "Optical Fabrication", pages 16-20. PA1 The Optical Society of America sponsored a topical meeting, Soft-X-Ray Projection Lithography Topical Meeting, on Apr. 10-12, 1991. A paper entitled "Design and Analysis of Multimirror Soft-X-Ray Projection Lithography Systems", by D. L. Shealy, C. Wang, and V. K. Viswanathan, was published in OSA Proceedings on Soft-X-Ray Projection Lithography, 1991, Vol. 12, Jeffrey Bokor (ed.), Optical Society of America, pages 22-26. PA1 Another paper on the subject, authored by R. H. Stulen and R. R. Freeman, and entitled "Optics Using a Laser Plasma Source", dated Nov. 15, 1990, was published in OSA Proceedings on Soft-X-Ray Projection Lithography, 1991, Vol. 12, Jeffrey Bokor (ed.), Optical Society of America, pages 54-57. PA1 A selection of overview papers from SPIE Proceedings-Summer/Fall 1990, SPIE Advent Technology Series, Volume AT 2, (ed., Western Washington University), SPIE Optical Engineering Press, includes, at page 320, a paper entitled "Design survey of x-ray/XUV projection lithography systems", by D. L. Shealy and V. K. Viswanathan. PA1 Another paper entitled "Reflective systems design study for soft x-ray projection lithography" is by T. E. Jewell, J. M. Rodgers, and K. P. Thompson, and appears in J. Vac. Sci. Technol. B 8 (6), Nov/Dec 1990, American Vacuum Society, pages 1519-1523.